Anode for brine electrolysis

ABSTRACT

An improved anode for the electrolysis of brine is comprised of a corrosion resistant valve metal substrate and a thin adherent exterior coating consisting essentially of ruthenium oxide and boron carbide.

United States Patent Inventors Carl D. Keith Summit; Alfred J. Haley, Jr., Florham Park; Robert M. Kero, Cranford, all of NJ.

Appl. No. 786,407

Filed Dec. 23, 1968 Patented Oct. 26, 1971 Assignee Engelhard Minerals 8: Chemical Corporation Newark, NJ.

ANODE FOR BRINE ELECTROLYSIS 6 Claims, No Drawings US. Cl 204/99, 117/230, 204/98, 204/129, 204/181 204/219, 204/250, 204/290 F Int. Cl C0ld 1/08 Field of Search 204/96, 98,

Primary Examiner-T. Tung Attorneys-Samuel Kahn and Miriam W. Leff ABSTRACT: An improved anode for the electrolysis of brine is comprised of a corrosion resistant valve metal substrate and a thin adherent exterior coating consisting essentially of ruthenium oxide and boron carbide.

ANODE FOR BRINE ELECTROLYSIS This invention relates to novel anodes for cells used for the electrolysis of brines, and more particularly to improved anodes comprised of platinum group metal coated electrolytic valve metals and a method for obtaining such anodes.

The anodes of the present invention are particularly useful in cells used for the production of chlorine and caustic soda by the electrolysis of an aqueous solution of sodium chloride. in such cells graphite anodes are usually used commercially. Although the graphite anodes are not entirely satisfactory because their wear rates are high and impurities such as CO are introduced in the products, no satisfactory substitutes have yet been found.

Platinum group metal coated electrolytic valve metals have been proposed as substitutes for graphite anodes. These metallic anodes offer several potential advantages over the conventional graphite anodes, for example, lower overvoltage, lower erosion rates, and higher purity products. The economic advantages gained from such anodes, however, must be sufficiently high to overcome the high cost of these metallic anodes. Anodes proposed heretofore have not satisfied this condition. Therefore commercialization of the platinum group metal anodes has been limited.

One problem is the life of the metallic anodes. A factor which contributes to shortening the anode life is the so-called undercutting effect. For economic reasons the low-overvoltage precious metal coatings are very thin films which are inherently porous. Although the electrolytic valve metals are substantially corrosion resistant, the valve metals are slowly attacked through the pores of these coating causing undercutting with subsequent loss of the precious metal film, thereby shortening the life of the anodes.

Another problem is the loss of precious metal during operation of the cell. Although the loss is gradual, it is costly because the precious metals are expensive and because the erosion of thin coatings shortens the anode life. The loss of precious metal may be from mechanical wear. At the highcurrent densities desirable in commercial installations, the increased fiow rate of brine and excessive gassing are conducive to such mechanical wear. ln mercury cells a contributing factor is amalgamation of the precious metals.

It is the object of this invention to provide metal electrolytic anodes with improved life and lower metal losses without sacrificing the low-overvoltage characteristics of the precious metal coating.

In accordance with the present invention a precious metal anode is provided which has long life and lower precious metal losses due to mechanical wear and amalgamation. The resistance to amalgamation makes the anode particularly useful in mercury cells. It was a further advantage of the anodes of this invention that the electrical properties were equal and even superior to conventional anodes using a greater equivalent weight of platinum group metals.

The anode of the present invention is comprised of a corrosion resistant metal substrate and a coating consisting essentially of ruthenium oxide and boron carbide.

The corrosion-resistant metal substrates, the so-called valve metals, used for electrolytic anodes are well known in the field. They are much less expensive than platinum group metals and they have properties which render them substantially corrosion resistant to the anodic environments in electrolysis cells. Examples of suitable corrosion-resistant valve metals are Ti, Ta, Nb, Hf, Zr, W, Al, and alloys thereof. It is also well known to have the valve metal as a layer on a base metal such as copper which is a good conductor but corrosive to the environment, and such modifications are within the scope of this invention.

Anodes of this invention are suitably prepared by depositing a slurry of boron carbide in the form of a fine powder in a liquid-medium containing ruthenium on a corrosion-resistant substrate and then firing the coating in an oxidizing atmosphere such as air to drive off the liquid and form an adherent coherent coating of ruthenium oxide and boron carbide. The coating may be deposited using the usual techniques such as by brushing, spraying or dipping. The coating may also be applied by electrophoresis. Suitably the boron carbide is present as a powder having a particle size of no greater than 250 microns. Preferably at least some of the boron carbide particles have a diameter no greater than about 10 microns. The ruthenium is present as a salt, oxide or the metal per se; it is present dispersed as a fine powder or dissolved in the aqueous or organic medium. When the slurry contains ruthenium as a salt or metal, the coated substrate is heated at a temperature in the range of about 400 to 800 C. to convert such ruthenium to ruthenium oxide and to form an adherent coherent coating. The time required to convert the ruthenium metal or salt to the oxide depends on the temperature used. Typically the coated substrates are fired in the air at 500 C. for 5 minutes; but longer firing times are also used. When the slurry contains ruthenium as ruthenium oxide, the coating is heated to higher temperatures, eg about l,000 C., for a period of time necessary to sinter the particles and form an adherent coherent coating.

Alternatively the ruthenium salt, oxide or metal is applied on the substrate followed by an application of the finely divided boron carbide and the coated substrate is fired as indicated above.

It should be understood that conversion of the ruthenium metal and salts may not be complete under the firing conditions given. Normally an equilibrium will be reached under which conversion is to predominantly ruthenium oxide and the balance ruthenium. Such materials are within the contemplation of this invention.

The concentration of ruthenium oxide in the coating ranges from about 5 percent to percent by weight and the boron carbide content ratio ranges from about 10 percent to percent by weight. It has been found that higher percentages of boron carbide increase the adherence of the coating without excessive sacrifice of the electrical characteristics. One preferred embodiment contains about 50 percent ruthenium oxide and 50 percent boron carbide.

Several applications of the dispersion may be deposited, preferably firing at the indicated temperature is performed after each application.

The following examples are given by way of illustration and not as a limitation of the invention. It will be appreciated that modifications within the scope and spirit of the invention will occur to those skilled in the art.

The examples show comparative tests in an electrolytic diaphragm cell using various anodes.

In each anode the substrate is a sheet of commercially pure titanium AXS X0063 inches. The titanium sheets are prepared for coating by etching in concentrated hydrochloric acid for 18 hours at a room temperature and cleaning in fluoboric acid.

Sample A is prepared as follows:

An aqueous paint composed (by weight) of 3.l3 percent boron carbide (milled to a fine particle size, 90 percent less than 8.2 microns and 50 percent less than 5.2 microns), 6.87 percent ruthenium chloride, and 90 percent water is applied to both sides of a previously prepared titanium sheet. The coated substrate is fired in air at 500 C. for 5 minutes. This procedure is repeated an additional 6 times to give a coating composed of 52.9 percent RuO, and 47.1 percent boron carbide (by weight). The total weight gain of the sample is 0.0263 grams. This deposit contains an amount of Ru equivalent in weight to a 17 microinch Ru coating. The deposit showed exceptionally good adherence and coherence.

Sample B is prepared as follows:

A low-overvoltage 70 percent Pt-30 percent It coating having a thickness of 27 microinches is applied to one side of a titanium sheet. The coating is applied from a paint using a known technique of application and firing.

Sample C is prepared as follows:

A low-overvoltage RuO layer having a thickness equivalent to 17 microinches of Ru, determined gravimetrically, is prepared from an alcohol-based paint containing RuCl: linalool and Z-propanol. The coating is converted to RuO, by heating in air at 500C. for minutes.

EXAMPLE 1 Samples A, B, and C are used as anodes in a laboratory scale diaphragm cell for the electrolysis of 25% NaCl solution. The test are seen at 35 C. and at a current density of 1,000 amperes per square foot (ASF). The chlorine overvoltage is determined with a conventional Luggin capillary probe, and

the results are shown in table I.

TABLE I Thickness of Precious Metal Chlorine Cell in Coating Overvoltage Potential Sample (microinches) (millivolts) (volts) The results in table I show that anode A, the anode of this invention, has excellent overvoltage properties; the performance of anode A surpassed that of anode C, which has a comparable and equivalent thickness of precious metal but does not contain the carbide, and even surpassed anode B, which has more than one and one-half the thickness of precious metal but does not contain the carbide.

An additional advantage of anode A is that for an equivalent weight of precious metal anode A is less expensive than anode 8 since Ru is less expensive than the Pt-Ir.

A still further advantage of anode A is that the presence of a carbide compound provides an effectively thicker coating for an equivalent weight of precious metalwithout the boron carbide and the presence of the carbide does not adversely affect the electrical characteristics and in fact shows improvement. in view of the greater thickness it could be expected that the coating would have longer life in commercial operation as iilustrated in the next exampie:

After 210 hours, sample C would not draw the specified density at its initial cell potential. Upon raising the celi potential rapid disintegration of both the coalinggd s da strate resulted. This demonstrates the superior life of the anode of this inventionv Sample A, an anode of this invention, was tested for an additional 200 hours at 3,000 a.s.f. without failure, indicating that the anode will last at least twice as long as sample C, which did not contain the carbide.

We claim:

1. An anode for the electrolysis of brines comprises of a corrosion resistant valve metal substrate and a thin adherent coating consisting essentially of ruthenium oxide and boron carbide.

2. An anode of claim I wherein the thin coating is composes of 5 to percent ruthenium oxide and 10 to percent boron carbide by weight.

3. An anode of claim I wherein the thin coating is composed of about 50 percent ruthenium oxide and 50 percent boron carbide.

4. An electrolytic cell comprised of a mercury cathode and an anode comprised of a corrosion resistant valve metal sub strate and a thin adherent coating consisting essentially of ruthenium oxide and boron carbide.

5. In the electrolysis of an aqueous alkali metal chloride solution in a cell having an anode by a process comprising contacting said solution with the anode and electrolyzing said alkali metal chloride solution by passing a current through said solution, the improvement wherein said anode is com prised of a corrosion resistant valve metal substrate and a thin adherent coating consisting essentially of ruthenium oxide and boron carbide.

6. A process of claim 5 wherein the cathode is comprised of mercury. 

2. An anode of claim 1 wherein the thin coating is composed of 5 to 90 percent ruthenium oxide and 10 to 95 percent boron carbide by weight.
 3. An anode of claim 1 wherein the thin coating is composed of about 50 percent ruthenium oxide and 50 percent boron carbide.
 4. An electrolytic cell comprised of a mercury cathode and an anode comprised of a corrosion resistant valve metal substrate and a thin adherent coating consisting essentially of ruthenium oxide and boron carbide.
 5. In the electrolysis of an aqueous alkali metal chloride solution in a cell having an anode by a process comprising contacting said solution with the anode and electrolyzing said alkali metal chloride solution by passing a current through said solution, the improvement wherein said anode is comprised of a corrosion resistant valve metal substrate and a thin adherent coating consisting essentially of ruthenium oxide and boron carbide.
 6. A process of claim 5 wherein the cathode is comprised of mercury. 